The present invention relates to a hydraulic dual-circuit brake system for a road vehicle equipped with an anti-lock system which works on the recirculation principle and with a device for driving-dynamics control which works on the principle of keeping the longitudinal and the lateral slip of the vehicle wheels within limits compatible, as a whole, with the dynamic stability of the vehicle by way of an automatic electronically controlled activation of one or more wheel brakes.
More particularly, the present invention involves a system with a device for the automatic initiation of full braking, which device works on the principle of activating the master cylinder of the brake system with a high actuating force by triggering the brake booster of the brake system, when the driver actuates the brake pedal at a speed (.phi.) which is higher than a threshold value (.phi..sub.s).
DE 4,208,496 A1 discloses one type of known brake system. In, for example, a known brake system which has a front-axle/rear-axle brake-circuit division, a tandem master cylinder actuable by pedal control by a pneumatic brake booster and having pressure outlets assigned individually to the brake circuits is provided as a brake-pressure control unit. The front-axle brake circuit I is connected to the primary outlet-pressure space of the tandem master cylinder which is limited movably on one side by the plunger-rod piston, and the rear-axle brake circuit II is connected to the secondary outlet-pressure space of the tandem master cylinder which is limited movably on one side by the floating piston of the tandem master cylinder. A pedal-position sensor generates electrical output signals which are characteristic of the particular instantaneous position of the brake pedal.
From the conventional processing of these output signals as desired-value signals, an electronic control unit provided for controlling the particular control mode (anti-lock control or driving-dynamics control) also recognizes the need for automatic full braking and initiates this full braking when the threshold value (.phi..sub.s) of the pedal-actuating speed is exceeded.
Furthermore, a pressure sensor issues output signals characteristic of the instantaneous value of the brake pressure generated during normal braking controlled by the driver, which signals are received as actual-value signals by the electronic control unit and are compared with the desired-value signals. Consequently, by utilizing the brake-pressure regulating valves of the anti-lock system, a desired-value-guided follow-up regulation of the brake pressure with the effect of a requisite brake-force distribution is achieved.
The selection of the wheel brake or wheel brakes to be activated in the driving-dynamics control mode and loadable with the pressure generatable as a result of the automatic activation of the brake booster or of the brake-pressure control unit takes place in that the inlet valves of the wheel brakes not to be loaded with brake pressure are switched into their shut-off position, and the inlet valve or inlet valves of that wheel brake or of those wheel brakes to be activated in the driving-dynamics control mode remains or remain held in the open position.
In the conventional brake system which, even in the event of a failure of the anti-lock system, allows a dynamically stable deceleration behavior of the vehicle, the critical vehicle deceleration (Z.sub.crit), which, according to the constructively predetermined installed front-axle/rear-axle brake-force distribution, corresponds to an equal adhesion utilization on the front wheels and the rear wheels when braking takes place on a dry non-skidding road, cannot be achieved without the cooperation of the driver in the generation of the actuating force acting on the plunger-rod piston of the tandem master cylinder, that is not already at the cut-off point of the brake booster. Therebeyond, the brake booster no longer contributes anything to the actuating force acting on the plunger-rod piston of the tandem master cylinder, and therefore a further increase in the brake force is possible solely by the driver exerted actuating force.
In practice, therefore, purely by cutting off the brake booster, only approximately 75% of that actuating force necessary to generate a maximum useful brake force can be exerted on the plunger-rod piston of the brake-pressure control unit. For the driving-dynamics control mode, this is disadvantageous inasmuch as the brake forces which can be generated purely by cutting off the brake booster are relatively far below the maximum brake forces which can be generated in the normal braking mode.
In a further brake system which is the subject of a previously unpublished Patent Application P 43 29 139.2, the foregoing disadvantage is avoided by utilizing, in the driving-dynamics control mode, the recirculating pumps of the two brake circuits as brake-pressure sources which, in as conventional piston pumps, are capable, in principle, of supplying very high outlet pressures. These outlet pressures are limited by pressure-limiting valves to values which are still sufficiently high to ensure that, even in the driving-dynamics control mode, the blocking limit can be reached on the individual wheel brakes and an optimum adhesion utilization of the individual braked vehicle wheels can be achieved by utilizing the anti-lock control functions. Brake fluid at a moderate pressure level of approximately 15 bar is delivered to the recirculating pumps on the inlet side by a precharging pump.
For the hydraulic separation of the two brake circuits, there are provided precharging cylinders which are each assigned individually thereto and which each have a piston which delimits in a pressure-tight manner an outlet-pressure space which can be connected, via an electrically triggerable precharging control valve, to the low-pressure connection of the recirculating pump of the respective brake circuit, relative to a drive-pressure space, to which the pressure outlet of the precharging pump is connected. This piston has two flanges which are arranged at an axial distance from one another and between which extends a follow-up space kept in constantly communicating connection with either one of the pressure outlets of the brake unit.
The piston of the respective precharging cylinder is provided with a mechanically actuable valve also performing the function of a non-return valve which shuts off a piston throughflow duct connecting the follow-up space to the outlet-pressure space, as long as the pressure in the outlet-pressure space of the respective precharging cylinder is higher than in the follow-up space, and which opens this throughflow path when the master-cylinder outlet pressure fed into the follow-up space is higher than the pressure in the outlet-pressure space of the precharging cylinder.
An electrically triggerable change-over valve is inserted in each case between the pressure outlet of the respective precharging cylinder and the main brake conduit of the brake circuit supplied with pressure thereby, and is opened when currentless and shuts off when in the energized state assigned to the DDC mode. The low-pressure inlets of the two recirculating pumps are connected to the main brake conduit of the respective brake circuit, each via a non-return valve which, as a result of a relatively higher pressure in the return conduit of the respective brake circuit than at the low-pressure inlet of its recirculating pump, is loaded in the opening direction and which otherwise shuts off. The two precharging cylinders have a common drive-pressure space which is connected to the pressure outlet of the precharging pump and which can be shut off relative to the brake-fluid reservoir of the brake system by triggering a currentlessly open solenoid valve.
The considerable technical outlay and space requirement which is necessitated by the two precharging cylinders and to which the precharging pump required additionally contributes must be considered as a substantial disadvantage of the brake-pressure control device according to aforementioned Patent Application P 43 29 139.2.
That brake system has a substantially more complicated construction and is also correspondingly more expensive in comparison with a vehicle equipped with a conventional anti-lock and traction control system which allows an automatic activation of only the wheel brakes of the driven vehicle wheels. On account of the additional precharging cylinders in a brake system thus far explained, there are also additional chambers, out of which gas bubbles, which can form in the case of a high thermal load on the wheel brakes, can escape only with difficulty. This is true particularly when the two precharging cylinders are combined to form a "symmetrical" constructional unit and therefore the arrangement of the two precharging cylinders which is most favorable for a passably good bleedability of these can be adopted at most with a view to the best possible compromise.
An object of the present invention is, therefore, to improve a brake system so as, regardless of the possibility of having the capacity to utilize high brake forces both in the normal braking mode and in the driving-dynamics control mode, to have a simpler construction and also to be less susceptible to faults.
This object has been achieved according to the present invention, by a system in which the brake-pressure control unit is configured to be controllable into a brake-pressure supply mode both by pedal-controlled pressure loading of a drive-pressure space of the associated brake booster and by valve-controlled pressure loading of tile drive-pressure space of the brake booster. The valve-controlled pressure loading is initiatable automatically by output signals from the electronic control unit. An auxiliary cylinder is provided for one of a supplementary and sole supply of brake pressure to the front-axle brake circuit of the brake system so as, by way of valve control, selectively loadable with and relieved of outlet pressure of an auxiliary pressure source and via which, both during normal driver-controlled partial or full braking and during automatically controlled partial or full braking, a brake quantity fluid is displaceable into the brake circuit connected to the primary pressure outlet of the brake-pressure control unit. The auxiliary cylinder constitutes a pressure intensifier whose maximum outlet-pressure level is sufficiently high for utilization of high adhesion coefficients in the driving-dynamics control mode. The brake-fluid quantity displaceable into the front-axle brake circuit is controllable in a predeterminable monotonic correlation from a comparative processing of output signals of the pedal-position sensor with output signals of the pressure sensor.
Accordingly, the brake-pressure control unit can be controlled in the brake-pressure supply mode both by the pedal-controlled pressure loading of a drive-pressure space of its brake booster and by the valve-controlled pressure loading, triggerable automatically by output signals from the electronic control unit, of the drive-pressure space of the brake booster. Only the rear-axle brake circuit is supplied with brake pressure solely by the brake-pressure control unit.
For a supplementary or sole supply of brake pressure to the front-axle brake circuit, an auxiliary cylinder can, by valve control, be loaded with and relieved again of the outlet pressure of a hydraulic pressure source present in any case on the vehicle, for example the supply pump of a power-assisted steering or a level control of the vehicle. Thereby, both during normal part or full braking controlled by the driver and during automatically initiated and controlled part or full braking, brake fluid can be displaced into the front-axle brake circuit which is connected to the primary pressure outlet of the brake-pressure control unit and which has the larger volumetric capacity.
This auxiliary cylinder is configured in the manner of a pressure intensifier. The maximum outlet-pressure level thereof utilizes high adhesion coefficients in the driving-dynamics control mode, that is to say can supply an outlet pressure. By way thereof, even during braked driving on a bend in which the highest wheel load occurs on the front wheel located on the outside of the bend, the latter can be braked up to the locking limit, so that the anti-lock control responds on this vehicle wheel. Provision is further made for the brake-fluid quantity displaceable into the front-axle brake circuit by the auxiliary cylinder to be controllable by a comparative processing of the output signals from the pedal-position sensor with the output signals from the pressure sensor in a predeterminable monotonic correlation with the output signals from the pedal-position sensor. As a result, the pedal-travel/brake-pressure characteristic of the brake system can be varied within relatively wide limits purely by electronic control. That is, in a brake system with a predetermined mechanical dimensioning of the brake-pressure control unit and of the wheel brakes, a desired pedal-travel/brake-pressure characteristic can be predetermined.
Presupposing a specific dimensioning of the brake-pressure control unit and of the auxiliary cylinder, brake systems can, with these control elements, for vehicles of differing weight and/or differing power be produced, inasmuch as there is the guarantee that it is possible to displace into the rear-axle brake circuit solely from the brake-pressure control unit a brake-fluid quantity which is sufficient to generate the maximum useful brake pressure in the rear-axle brake circuit. The brake-fluid volume sufficient for generating the maximum useful brake pressures in the front-axle brake circuit can be provided by the auxiliary cylinder in combination with the brake-pressure control unit.
In addition, the brake-pressure control unit can be operated, in relation to the maximum brake pressure to be built up, with a relatively "small", low-rated, brake booster. Thus, for example, in instances in which, with the conventional design of the brake system, a pneumatic brake booster would be necessary in a tandem form of construction, a single-stage pneumatic brake booster can be sufficient. For example, so that the same brake-pressure control unit can be used in each case within a production series of vehicles, the vehicle types of which differ merely in being equipped with engines of different power, it will be sufficient if the auxiliary cylinder can feed into the front-axle brake circuit between 30% and 70% of that brake-fluid quantity which has to be displaceable into the latter in order to make it possible to generate in the front-wheel brakes the brake pressure necessary for reaching the locking limit of the braked wheels.
In such a configuration of the brake system, if the brake booster fails, virtually the maximum possible vehicle deceleration can still always be achieved, as long as the auxiliary cylinder is operative. If the auxiliary cylinder fails, approximately two-thirds (2/3) of the maximum possible brake force can still be generated. If one of the two brake circuits fails, the other brake circuit remains operative, a failure of the front-axle brake circuit leading to a smaller idle travel of the brake pedal than in a conventional brake system.
This applies particularly to the configuration of the brake-pressure control unit and of the auxiliary cylinder of the brake system according to the present invention, in which, if the front-axle brake circuit fails, only a small idle travel of the brake pedal occurs until the brake system responds.
The brake system of the present invention allows the driving-dynamics control to be implemented by the use of the hydraulic unit of a standard four-channel anti-lock system. Only the electronic control unit has to be designed (programmed) for the additional control function.
An implementation of the driving-dynamics control function in the brake system according to the present invention utilizes as a pressure source for the rear wheel brakes which, in the driving-dynamics control mode and also in the anti-lock control mode assuming that the vehicle has a rear-axle drive, the recirculating pump of the rear-axle brake circuit which, at its low-pressure inlet, is loaded with the outlet pressure suppliable by the secondary outlet of the brake-pressure control unit which to that extent is operated as a precharging-pressure source. The brake-fluid delivery to the recirculating pump takes place via a supply control valve. The main brake conduit of the rear-axle brake circuit is shut off relative to the secondary pressure outlet of the brake-pressure control unit by a change-over valve. This has the advantage that the brake pressure, which can be fed into the rear-wheel brakes and by means of which the output-torque fraction of the vehicle engine, effective on the respective driven vehicle wheel, also still has to be compensated or "braked away" can, if necessary, be increased well beyond the brake pressure which would be achievable solely by the automatic activation of the brake-pressure control unit.
For as rapid a brake-pressure build-up as possible in the wheel brake or wheel brakes to be activated of the rear-axle brake circuit, it is advantageous at the same time if a non-return valve is connected parallel to the change-over valve and, as a result of a higher pressure at the secondary-pressure outlet of the brake master cylinder than in the main brake conduit of the rear-axle brake circuit, is loaded in the opening direction and otherwise shuts off. Thus, the particular higher pressure prevailing at the outlet of the brake master cylinder or generated by the recirculating pump is fed into the main brake conduit of the rear-axle brake circuit, which, at the start of the brake-pressure build-up, will usually be the outlet pressure of the brake-pressure control unit. Such a non-return valve also ensures that braking can still take place in the event of a malfunction of the change-over valve, for example retention in each shut-off position.
In order reliably to avoid an excessive generation of pressure in the rear-axle brake circuit, it is advantageous if the change-over valve which, in its switching position, shuts off the main brake conduit of the rear-axle brake circuit relative to the secondary-pressure outlet of the brake-pressure control unit, performs the function of a pressure-limiting valve in this switching position, or if there is connected parallel to the change-over valve a pressure-limiting valve, by way of which the rear-axle brake pressure is limited to a sufficiently low value of, for example, 170 bar.
In the brake system of the present invention, a pressure reduction in the course of automatically controlled braking is also possible in that, via the pressure-limiting valve, the recirculating pump conveys brake fluid back into the master cylinder (the brake-pressure control unit) while at the same time the precharging control valve of the rear-axle brake circuit is held in its basic shut-off position, whether as a result of a malfunction or intentionally. This utilization of the recirculating pump for pressure reduction is intended preferably for the emergency situation in which the change-over valve is caught in its shut-off position and the precharging control valve falls back into its basic shutoff position as a result of a malfunction. This malfunction can be recognized by the electronic control unit and the fact that there is a marked wheel slip on at least one of the rear wheels, although a signal combination which would be characteristic of a situation requiring control is not given.
If the brake pressure in the rear-axle brake circuit is adjustable, as it were, independently of the pressure which can be fed into the front-axle brake circuit, a pressure sensor can advantageously monitor the pressure in the main brake conduit of the rear-axle brake circuit and delivers its electrical output signals to the electronic control unit as information input signals which can then be utilized, both during normal braking and during automatically controlled braking, to control a situation-specific-brake-pressure/time curve.
A brake-pressure generation in the front-axle brake circuit, which is strictly correlated to the output pressure of the brake-pressure control unit, but, in principle, can be controlled as desired, and which can be advantageous for an electronically controlled brake-force distribution control, is made possible in a simple way by providing an outlet control valve constituting a solenoid valve and configured to be changeable over from a basic position, in which the primary pressure outlet of the master cylinder is connected to the pressure-supply connection of the main brake conduit of the front-axle brake circuit, into a shut-off position.
It is also advantageous, at least in combination therewith, if the recirculating pump of the rear-axle brake circuit is a so-called self-priming pump which, in the event of a need for traction control or driving-dynamics control, allows activation of at least one of the rear-wheel brakes of the vehicle, even when the brake-pressure control unit is not triggered.
As a result of the configuration of the auxiliary cylinder in accordance with the present invention as described in detail below, a reliable media separation is achieved in the event that the auxiliary-pressure source, with the outlet pressure of which the drive stage of the auxiliary cylinder can be loaded, is operated with a hydraulic oil, for example a conventional mineral oil which should not be mixed with brake fluid if packing damage to the piston packings of the displacement piston and/or of the drive piston of the auxiliary cylinder is to be reliably avoided.
The configuration of the drive-piston stage and of the displacement-piston stage of the auxiliary cylinder as separate piston elements has the additional advantage that production tolerances of the auxiliary cylinder housing, i.e., slight eccentricities of the bore stages in which the piston elements are displaceable in a pressure-tight manner, can be compensated in a simple manner. The same advantage applies if the housing of the auxiliary cylinder is of multi-part configuration and its piston elements are arranged in different housing parts. Such a multi-part configuration of the auxiliary cylinder can be expedient so that a displacement part of the auxiliary cylinder can be coupled to differently dimensioned drive parts, in order thereby to achieve an adaptation to the respective outlet-pressure level of various auxiliary-pressure sources.